Download SECTION D What Does DNA Do?

CHAPTER 1
DNA: The
Hereditary
Molecule
SECTION D
What Does
DNA Do?
Chapter 1 • Modern Genetics for All Students
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D.1
DNA Codes For Proteins
PROTEINS DO THE nitty-gritty jobs of every living cell. Proteins are the molecules that
give structure and shape to living cells and that carry out all of the chemical reactions necessary for life. The importance of DNA is that it contains the information that is used to
make all of the proteins on which life depends.
The proteins whose structures are specified by our DNA do more than carry out all of the
reactions necessary to keep our cells alive; they also digest our food for us, influence how
we will respond to infections, and determine what color our eyes, hair and skin will be.
Proteins are made of long strings of individual building blocks known as amino acids.
Amino acids come in 20 different kinds that are all slightly different from one another
chemically. It is not important for us to understand the details of the chemical differences
that distinguish these 20 kinds of amino acids. But it is important to realize that because of
these differences, the structure and function of every protein depends on the sequence in
which the various kinds of amino acids are strung together. A protein may contain many
hundred amino acids. But if just one of these amino acid is changed, the function of the
protein might change drastically.
A gene, which is a functional unit of DNA, carries coded information indicating the precise sequence in which amino acids should be strung together to make one particular kind
of protein that will play one particular role in the life of the cell.
The elongated shape of collagen
protein, shown in this molecular
model, allows collagen to provide
structural support to cells and
organs.
Chapter 1 • Modern Genetics for All Students
Antibodies, such as this human
immunoglobulin, recognize and
attack viruses, bacteria, and
other foreign substances.
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D.1
There are several different categories of proteins. Most proteins function as enzymes to
regulate the speed of particular chemical reactions. Every cell contains hundreds of different enzymes that work together to release energy from food molecules and to use energy to
build new cellular materials. All enzymes are proteins. But not all proteins are enzymes.
Structural proteins, as the name implies, provide structural support inside and outside
cells and for the body as a whole. Defense proteins are an assortment of proteins in the
blood that recognize and fight off foreign invaders, such as bacteria and viruses.
Transport proteins are used to carry various molecules into and out of cells as well as
through the body. The table on the next page gives a few examples of these and other
kinds of proteins.
Every aspect of life depends on proteins. In the next set of activities, you will learn about
the processes a cell uses to convert the coded information in a bit of DNA (a gene) into a
particular kind of protein.
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D.1
A FEW HUMAN PROTEINS AND THEIR FUNCTIONS
Protein
Type
What it does
hemoglobin
a red, iron-containing,
transport protein
• makes your blood red
• picks up oxygen from the air in your lungs
and releases it to cells elsewhere in your body
• if you don’t make enough hemoglobin, or you
make a type that doesn’t carry oxygen well,
you will be anemic
insulin
a hormone
• tells your cells when to remove sugar from
your blood after a meal
• if you don’t make enough insulin, or if your
cells do not respond to it well, the
concentration of sugar in your blood will get
too high, and you will have diabetes
antibody
a defense protein
• fights infections from bacteria and viruses
• vaccination triggers production of antibodies
that will fight a particular kind of infection
• if you couldn’t make any antibodies, you
would have been dead long ago!
lactase
an enzyme in your digestive tract
• digests lactose (milk sugar) in milk and dairy
products into simpler sugars
• if you don’t make enough lactase, you get
diarrhea when you drink milk or eat ice
cream or other diary products
• but you can buy lactase pills to swallow
before eating dairy products
collagen
a structural protein
• provides the tough structural framework of
your skin, bones, tendons, and cartilage
keratin
a structural protein
• provides the resistant outer layer of your skin
and the tough structural material of your hair
and nails
myosin
a structural protein
that is also an enzyme
• uses energy derived from food — and works
together with actin — to cause muscles to
contract
• you can increase the myosin content of your
muscles with exercise
Chapter 1 • Modern Genetics for All Students
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D.2
How DNA Codes For Proteins
Reprinted with permission from Gonick, L. and Wheelis, M. (1991).
The Cartoon Guide to Genetics. New York: Harper Collins Publishers
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D.2
Name __________________________________________________
Date ____________________________ Hour ________________
QUESTIONS ON “HOW DNA CODES FOR PROTEINS”
AFTER YOU HAVE read about protein synthesis in your excerpt from the Cartoon Guide
to Genetics, answer the following questions.
1. What is the relationship between genes and proteins? ___________________________
______________________________________________________________________
2. How does RNA differ from DNA? __________________________________________
______________________________________________________________________
3. What is the molecule that carries the information from a gene to the place
where a protein will be made? _____________________________________________
4. What is the process by which such a molecule is made? _________________________
5. What is the enzyme that mediates the process named above? _____________________
6. What is the structure on which proteins are made? _____________________________
7. How many bases form one “word” of the RNA message? _______________________
8. What is the technical name for such a group of bases found on mRNA? ____________
9. What is another term for protein synthesis? ___________________________________
10. What is the group of molecules that translates the genetic code? __________________
11. What is an anticodon? ___________________________________________________
12. At the tail end of each tRNA molecule, an ________________ attaches the appropriate
____________________________ molecule to the tRNA.
13. What happens when two tRNAs are side by side on a ribosome? __________________
______________________________________________________________________
14. The first codon on an mRNA always is ______________________________________.
15. This codes for the amino acid called ________________________________________.
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D.3
The Gene Expression Dance
AS YOU HAVE READ in your excerpt from the Cartoon Guide to Genetics, three kinds
of RNA molecules cooperate to convert the coded information in DNA (a gene) into a protein with a particular sequence of amino acids.
The process in which the nucleotide sequence of a gene (DNA) is used to specify the
amino acid sequence of a protein is called gene expression, and it consists of two major
phases: 1) transcription, in which a messenger RNA (mRNA) molecule that is complementary in nucleotide sequence to the gene is synthesized, and 2) translation, in which
the message carried by that mRNA molecule is used to synthesize the corresponding protein. In this exercise, your class will perform a simulation of these two processes.
MATERIALS
Your teacher will hand out the gene-expression flash cards that you will use in this dance.
PROCEDURE
Your teacher will instruct you how to perform the dance.
DNA
A
mRN
C
G
T
G
G
G
mRNA
4
DNA
A
DN 2
C
A
Meth
A
co
T
T
A
U
G
T
G
G
ionin
e
o
r
p
Isoleucine
de
s
U
A
n
i
te
U
DN
U
U
T
G
7
A
G
U
G
mRNA
Histidine
U
A
A
Glutamine
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D.4
Name __________________________________________________
Date ____________________________ Hour ________________
Paper Proteins: Models for
Simulating Gene Expression
MATERIALS
For each student or pair of students:
1 set of paper-protein puzzle pieces
PROCEDURE
A
Follow the directions below to model
the processes of transcription and translation, and to make a paper protein.
DNA
T
G
C
G
T
G
A
G
T
T
T
A
A
C
T
C
C
T
C
C
A
G
G
Figure 1
1. Place the DNA strip on the desk so
that the letters read properly for you
(fig. 1).
mRNA
2. Working from left to right, find the
mRNA pieces that match the DNA
and line them up (fig. 2). What is
the process in which an mRNA molecule that is complementary to a
DNA molecule is produced?
A
DNA
T
G
C
G
T
G
A
G
T
T
T
A
A
C
Figure 2
____________________________
(Answer question 2 here)
3. Separate the mRNA from the DNA
to simulate the mRNA moving out
of the nucleus to a ribosome in the
cytoplasm of the cell. Leave the
mRNA pieces lined up next to one
another(fig. 3).
mRNA
C
C
U
A
G
A
A
C
U
C
A
A
G
U
U
Figure 3
4. Match each of the tRNA pieces to
the amino acid piece that fits with it tRNA
(fig. 4). Lay them out so that all of
them are visible.
Amino
Acid
A
U
C
Methionine
Figure 4
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D.4
Name __________________________________________________
Date ____________________________ Hour ________________
5. Search for the tRNA that will base
pair with the first codon of the
mRNA
mRNA (the one on the left-hand
end). Move the tRNA with its
tRNA
attached amino acid into place in the
Amino
mRNA (fig. 5). Continue with the
second codon, and so forth. What is Acid
the name for this process in which a
protein is produced that has an
amino acid sequence specified by an
mRNA molecule?
U
A
C
C
U
A
G
A
A
C
U
C
A
A
G
U
U
A
G
G
C
Methionine
Figure 5
____________________________
(Answer question 5 here)
6. Now pull the string of six amino
acids away from the tRNA (fig. 6).
These six amino acids represent a
new protein. (However, real proteins Amino
Acid
always contain many more amino
acids than this, sometimes more
than a thousand.)
Methionine
Figure 6
After you have completed these steps, use the model pieces to explain the two component
processes of gene expression to another student. Then write out the steps in your own
words. You may refer to your notes or a book to check for scientific accuracy.
________________________________________________________________________
________________________________________________________________________
________________________________________________________________________
________________________________________________________________________
________________________________________________________________________
________________________________________________________________________
________________________________________________________________________
________________________________________________________________________
________________________________________________________________________
Chapter 1 • Modern Genetics for All Students
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Name __________________________________________________
D.5
Date ____________________________ Hour ________________
Using the Genetic Code to
Translate an mRNA
AT THE HEART OF the regular and predictable relationship between the sequence of
nucleotides in any gene and
the sequence of amino acids
present in the protein for
which that gene is said to
“code” is a fixed set of
nucleotide-to-amino acid
relationships that is known as
the genetic code. Just as the
Morse code can be printed in
the form of a table indicating
which letter of the English
alphabet is specified by each
combination of dots and
dashes, the genetic code is
usually printed in the form of
a table indicating what kind
of amino acid is specified by
each possible mRNA codon.
To the right is a DNA coding
sequence that codes for part
of the hemoglobin molecule.
Complete the mRNA strand
following base-pairing rules.
Then use the mRNA Genetic
Code Table on the next page
to determine the amino acid
sequence for which this piece
of a gene codes.
DNA
mRNA
Amino Acid
T
A
C
C
A
G
G
T
A
A
A
C
T
G
T
G
G
G
C
T
T
C
T
C
T
T
T
A
G
T
C
G
C
C
A
T
Congratulations! You have just “synthesized” the first part (12
amino acids) of the protein called ß-globin, which is part of the
hemoglobin molecules that make your blood red and carry
oxygen to cells throughout your body. (Each real ß-globin molecule actually consists of a string of 147 amino acids.)
Chapter 1 • Modern Genetics for All Students
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D.5
1st base
3rd base
mRNA GENETIC CODE TABLE
U
U
UUU = phe
UCU = ser
UAU = tyr
UGU = cys
U
UUC = phe
UCC = ser
UAC = tyr
UGC = cys
C
C
2nd base
A
G
Amino Acid
Abbreviations
ala
arg
asn
asp
=
=
=
=
alanine
arginine
asparagine
aspartic acid
cys = cysteine
C
UUA = leu
UCA = ser
UAA = stop UGA = stop A
UUG = leu
UCG = ser
UAG = stop UGG = trp
G
CUU = leu
CCU = pro
CAU = his
CGU = arg
U
CUC = leu
CCC = pro
CAC = his
CGC = arg
C
CUA = leu
CCA = pro
CAA = gln
CGA = arg
A
CUG = leu
CCG = pro
CAG = gln
CGG = arg
G
gln
glu
gly
= glutamine
= glutamic acid
= glycine
his
= histidine
ile
= isoleucine
leu
lys
= leucine
= lysine
met = methionine
A
G
AUU = ile
ACU = thr
AAU = asn
AGU = ser
U
AUC = ile
ACC = thr
AAC = asn
AGC = ser
C
AUA = ile
ACA = thr
AAA = lys
AGA = arg
A
AUG = met
ACG = thr
AAG = lys
AGG = arg
G
GUU = val
GCU = ala
GAU = asp
GGU = gly
U
GUC = val
GCC = ala
GAC = asp
GGC = gly
C
GUA = val
GCA = ala
GAA = glu
GGA = gly
A
GUG = val
GCG = ala
GAG = glu
GGG = gly
G
Chapter 1 • Modern Genetics for All Students
phe = phenylalanine
pro = proline
ser
= serine
thr
trp
tyr
= threonine
= tryptophan
= tyrosine
val
= valine
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